Our long-term objective is to elucidate the endocytic trafficking pathways of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) in order to identify a therapeutic target for the fatal disease, cystic fibrosis (CF). CFTR is expressed in the apical plasma membrane in epithelial cells where it functions as a cAMP-activated Cl- channel. CFTR mediated Cl- transport across polarized epithelial cells is regulated by modulating channel activity and by controlling the number of CFTR channels in the plasma membrane. ?F508, the most common mutation in CF, reduces the number of CFTR channels in the plasma membrane because ?F508-CFTR is not efficiently exported from the endoplasmic reticulum and because the plasma membrane half-life of ?F508-CFTR is reduced. The mechanism of reduced plasma membrane half-life of ?F508-CFTR is not completely understood, in part, because the protein interactions that facilitate the endocytic trafficking of CFTR at the plasma membrane have not been completely elucidated. In preliminary studies we identified several proteins that regulate trafficking of CFTR at the apical membrane in human airway epithelial cells. Elucidating the role of these proteins in CFTR trafficking will be critical for understanding the apical membrane trafficking defect of ?F508-CFTR. Accordingly, we propose to test the hypothesis that the airway cell apical membrane density of WT-CFTR and ?F508-CFTR is differentially regulated by protein interactions that occur during their internalization from the apical membrane, trafficking along the endocytic pathway, and sorting for either recycling or degradation. To test this hypothesis we propose three specific aims: Specific Aim #1. Test the hypothesis that Dab2 inhibits the expression of CFTR in the apical membrane by facilitating CFTR endocytosis in airway epithelial cells. The goal of this specific aim is to elucidate the role of Dab2 in CFTR endocytosis and to examine whether the ?F508 mutation accelerates the Dab2 mediated endocytosis of CFTR. Specific Aim #2. Test the hypothesis that c-Cbl inhibits the expression of CFTR in the apical membrane by facilitating CFTR endocytosis in airway epithelial cells. The goal of this specific aim is to elucidate the role of c-Cbl and its adaptor protein, CIN85 in CFTR endocytosis and to determine whether the ?F508 mutation accelerates the c-Cbl mediated endocytosis of CFTR. Specific Aim #3. Test the hypothesis that Rab4 inhibits the expression of CFTR in the apical membrane by sorting the internalized CFTR for lysosomal degradation in airway epithelial cells. The goal of this specific aim is to elucidate the role of Rab4 in targeting internalized CFTR for degradation and to determine whether the ?F508 mutation accelerates the Rab4 mediated sorting of internalized CFTR. We anticipate that our studies, performed in human airway epithelial cells: (1) will expand our understanding of the endocytic trafficking of CFTR; (2) will elucidate the mechanism of decreased plasma membrane half-life of ?F508-CFTR; and (3) will lead to a new therapeutic approach in patients with CF. PUBLIC HEALTH RELEVANCE. Cystic Fibrosis (CF) is an inherited disease that affects one in every 2,500 children born in the US. The disease affects breathing and digestion and there is currently no cure for the disease. CF patients cannot move salt (sodium chloride) into and out of certain cells, including those that line the lungs and pancreas and as a result produce thick, sticky mucus and other secretions. The long-term goal of this application is to develop a drug that will restore salt movement into and out of cells and alleviate the symptoms in CF patients.